1. Field of the Invention
The invention relates to a starting circuit for a motor, and particularly to a starting circuit for a single-phase AC motor.
2. Description of the Related Art
Single-phase AC motors are widely used in home appliances and industries such as refrigerators, freezers, washing machines, dryers, fans, pumps, and so on. In a three-phase AC motor, after being powered on, a stator winding generates a rotating magnetic field and induces a rotor to rotate. Conventional single-phase AC motors can only generate magnetic field with alternating polarities and strength instead of a rotating magnetic field. Namely, a single-phase AC motor cannot start itself.
To solve the above problem, a single-phase AC motor with a squirrel-cage rotor and a wound stator having a pair of windings (a main winding as a working winding and an auxiliary winding as a starting winding) is used, and the starting winding is connected to a centrifugal switch.
During startup, AC voltage is supplied to the main winding and the starting winding. After the speed reaches 70-85% of a synchronous speed, the centrifugal switch switches off the starting winding and the main winding continues operating. The phase difference between the current of the starting winding and that of the main winding is approximately 90 degrees, and the stator generates rotating magnetic field in air gaps. However, the mechanical contacts in the centrifugal switch often generate electric sparks, which reduce the lifetime and reliability of the centrifugal switch. Moreover, the centrifugal switch is usually disposed in the motor, and thus increases cost and size of the motor. Accordingly much opportunity for improvement remains.